In a first aspect, the invention relates to antennas for hand-held, portable, mobile, marine, or fixed wireless communications devices (WCD""s), including, for example, hand-held, notebook, or desktop computers, cellular telephones, data devices, communications transceivers, global positioning satellite (GPS) receivers, and vehicular digital radios. In particular, a first aspect of the invention relates to an antenna which includes two crossed driven elements in proximity to a ground plane arranged so as to exhibit dual-linear and circular (or elliptical) polarizations simultaneously and provide at least a substantially hemispherical antenna pattern. In compact embodiments, the antenna easily fits inside the plastic housing of a WCD, thereby providing mechanical robustness. The antennas according to the present invention may be used for transmitting, receiving, or for transmitting and receiving.
In a second aspect, the invention relates to precision capacitors for use in antennas of the type that form the first aspect of the invention and for use in other types of antennas.
In its first aspect, the present invention relates to antennas that include a conductive ground plane with two bent quarter-wave (electrically) crossed driven elements fed ninety degrees out of phase arrayed over at least a portion of it. Each driven element is electrically substantially a quarter wavelength at or near a desired operating frequency or within a desired operating frequency band. Each driven element preferably has generally a U-shape, having bent-over end portions generally perpendicular to the ground plane and a central portion generally parallel to the ground plane. At least one end of each element is electrically coupled to the ground plane (directly or through a resonating capacitor that allows the element to be electrically a quarter wavelength but physically less than a quarter wavelength). The central portions cross at about ninety degrees without touching and without being so close as to substantially capacitively couple to each other. The parallel portions of the driven elements are closely spaced to the ground plane. The driven elements may be directly end fed or shunt fed, in each case with or without a matching network. In practical embodiments of the U-shaped configuration, operating bandwidths of approximately two to ten percent can be achieved.
Alternatively, and less desirably (because the antenna""s patterns are degraded relative to those resulting from U-shaped driven elements), each driven element may have a single vertical portion and a portion that slopes from the top of the vertical portion to toward the ground plane, thus having an overall L-shape (but with an angle of less than ninety degrees). Either the end of the single vertical portion or the other end is electrically coupled to the ground plane (directly or through a resonating capacitor). The other end may be directly end fed or electrically coupled to the ground plane and shunt fed.
The overall length and height above the ground plane of the two driven elements may be varied simultaneously to vary gain, with longer length and greater heights producing higher gain. A dielectric, with a dielectric constant greater than one, may be located inside the volume or xe2x80x9ccagexe2x80x9d formed by the crossed elements to reduce the height of the elements above the ground plane. In some embodiments of the first aspect of the invention, a molded dielectric is located within the crossed elements. The dielectric may be plastic or some other moldable material having suitable dielectric characteristics such as fiberglass or a ceramic material.
In the U-shaped driven element configuration, the portions of the driven elements perpendicular to the ground plane cause a linear polarization, and the portions parallel to the ground plane cause circular or elliptical polarization. A further linear polarization is caused by the major dimension of the ground plane. Useful radiation is exhibited at either a linear or a circular (or elliptical) polarization in substantially a hemisphere over the side of the ground plane over which the driven elements are arrayed. Configurations employing the modified L-shaped driven elements will exhibit polarizations similar to those just mentioned, but somewhat degraded.
The circular (or elliptical) polarization, when it is right-handed circular polarization (RHCP), is particularly useful for receiving RHCP signals from GPS satellites. The orientation of the ground plane with respect to the zenith is not critical for this application, which makes the antenna of this invention particularly useful when, for example, it is mounted near the top of a WCD such as a cellular telephone that may be held by the user in a variety of positions. In practical embodiments, the antenna of this invention, when optimally oriented and connected to a GPS receiver, can provide lock-on to the GPS system in a time comparable to a quadrifilar helix, which typically is a much larger antenna.
To provide a compact realization of an antenna according to the first aspect of the invention, a capacitor may be provided at the end of the element or within the element to series resonate the element to a desired frequency and reduce its physical length. A driven element thus can be reduced to much less than a physical quarter wavelength. In order to provide substantial resonance within a desired range of frequencies, the capacitance must be set and maintained within a predetermined range with a high degree of accuracy. Each xe2x80x9cresonatingxe2x80x9d or xe2x80x9ctuningxe2x80x9d capacitor may be located between the end of the element and the ground plane (this location is easier to implement and is shown herein in the exemplary embodiments) or, alternatively, each may be located in series along the element within about the half of the element closest to the second end of the driven element such that it splits the element into segments, a segment between its first end and the capacitor and the segment between the capacitor and its second end.
The second aspect of the present invention relates to precision capacitors that are particularly useful in antennas according to the first aspect of the invention and also for use in other antennas, including, but not limited to, physically-shortened single element bent quarter-wave antennas. Off-the-shelf capacitors and capacitors employing PWB dielectrics generally are not satisfactory for one or more reasons: they may not provide the required tolerance and precision (for example, the thickness of PWBs varies, the capacitance may vary with temperature), they may not be available in the required capacitance value, and they may not be available with the required tolerance. Typically, off-the-shelf chip capacitors are available with values starting at about 0.1 pf (picofarads) in increments of 0.1 pf with tolerances of about plus/minus 0.1 pf. In this application, non-standard capacitance values are required with tolerances of about plus/minus 0.05 pf to assure continued resonance of a shortened driven element within a desired frequency band during normal operation.